Distortionless amplifier



June l2, v1934- E. oosTERHuls E-'r'AL f 1,952,458

DIsToRTIoNLEss AMPLIFIERy l Filed July 7. 1930Y f mum lit* if www x'mxxxxxx INVENTORS Emo OosTERHuxs CORNELI E RlK MOREL BY Ao-RNEY Patented June 12, 1934 1 1,962,458 DIs'roRTIoNLEss AMPLIFIER Ekko Oosterhuis and Cornelis Hendrik Morel, Eindhoven, Netherlands, assignors to Radio Corporation of America, a corporation of Delaware Application July 7, 1930, Serial No. 465,949 In the Netherlands August 23, 1929 16 Claims. (Cl. 179-171) The present invention relates to amplier circuit arrangements, and more particularly to amplifiers Vof the type wherein a rather broad band of frequencies, after being amplified, are transmitted, as is the case, for example, with audio frequency amplifiers.

The anode circuits of the thermionic devices pertaining to such an amplifier usually comprise the primary winding of a transformer, or the winding of a reproducer, for example Va loud speaker; in both cases, consequently, an inductive load, the impedance of which increases with the frequency. If the tube in question is of a type wherein the high tones are also, or just, strongly transmitted, it frequently happens that, as a consequence thereof, in connection with the aforementioned large impedance for the high frequencies, considerable distortion of the high tones occurs, the distortion being probably due to an 0 excessive periodic decrease of the anode potential.

According to this invention, this defect is eliminated by connecting a resistance with the impedance included in the anode circuit of the tube concerned. In this case, the impedance can never exceed the value of the said resistance. If the anode circuit comprises a transformer, the resistance may be connected with the secondary winding, as well as with the primary.

The novel features which we believe to be char acteristic of our invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which we have indicated diagrammatically several circuit organizations whereby our invention may be carried into effect.

In order that the invention will be more clearly understood and readily carried into effect, the accompanying drawing represents, by way of example, four embodiments of a multi-tube amplier in which the invention is only applied to the output circuit of the end tube.

In the drawing,

Fig. 1 `represents a circuitarrangement in which the resistance is shown connected in parallel with the primary winding of the output transformer,

In Fig. 2 the resistance is shown connected in parallel with the secondary winding of the output transformer,

In Fig. 3 the resistance is shown in parallel with the loudspeaker,

Fig. 4 shows a modiiication of the circuit in Fig. 2.

The circuit arrangement represented in Fig. 1, is employed for the ampliiication of audio currents supplied to the primary winding 1 of an input transformer, the secondary winding 2 of the latter1 being located between the cathode and the control grid 3 of the rst amplifier tube V1. The anode 4 of this tube is connected through the primary winding 5 of a second transformer to the positive terminal of a source of anode current (not shown). The secondary winding 6 of this transformer is located between the control grid '7 and the cathode of the second, and last, tube V2.

The latter is preferably an electron discharge tube of the pentode type comprising a screen grid 8 and an auxiliary grid 9 located between the screen grid and the anode, said auxiliary grid being directly electrically connected to the middle of the cathode. The anode l0 is connected through the primary winding 11 of an output transformer to the abovementioned anode positive terminal. f

In parallel with the winding 11 is connected a resistance 12 which may be chosen of the order of magnitude of 10,000 to 100,000 ohms inclusive. Good results have been obtained, for example with a resistance of 40,000 ohms. These values are, of course, illustrative, and not limiting. Finally, the secondarywinding 13 of the output transformer is connected to a utilization means, such as head phones, a loud speaker or the like.

The live electrode, or pentode, tube V2 has, among others, the property of amplifying high frequencies, under for the rest equal conditions, much more intensely than do the commonly used triodes. That is to say, a pentode tube has a rising frequency amplification characteristic. Since the impedance of an inductive load such as a loud speaker or a transformer winding, increases with the frequency, loud high tones amplified by a pentode produce in such a winding such a great drop of potential, that the potential variations on the anode reach an undesirably high value, and sound distortion occurs. Now, the resistance 12, connected in parallel with the inductive load of the pentode, limits the value of the output impedance, and, therefore, the value of the voltage variations on the anode of the said pentode.

ico

The circuit arrangements shown in Figs. 2 and i 3, are analogous to that of Fig. 1 with the difference, however, that in Fig. 2 the resistance 12 is connected in parallel with the secondary winding the resistance 12 is connected to the winding of the loud speaker 14, which, without the coupling of an output transformer, is directly connected into the anode circuit of the pentode tube. As above noted, the invention is in the first place of importance in conjunction with amplifier tubes whereby the high frequencies are strongly transmitted. The advantage of the invention is particularly conspicuous when the negative grid bias of such a tube is taken off in the known and usual manner from a resistance traversed by the anode current of the tube itself.

Such an arrangement is shown in Fig. 4, which for the rest is quite analogous to Fig. 2. In this case the control electrode 7 is connected through the secondary transformer winding 6 to the negative terminal of the anode source l5, and, also, to one of the ends of the resistance 16 the other end of which is connected to the mutually connected cathodes of the valves V1 and V2. A negative bias is, thus, applied in the known manner to the control electrode 7, said bias being equal to the drop of potential produced in the resistance 16.

With this method the control grid potential experiences a strong reaction of the current variations in the anode circuit, the percentage value of these variations being reduced by using a resistance in parallel with the output impedance. By a suitable choice of the value of the said resistance, it is, therefore, possible to keep the final sound undistorted even in the case of loud high tones.

While we have indicated and described several systems for carrying our invention into effect, it will be apparent to one skilled in the art that our invention is by no means limited to the particular organizations shown and described, but that many modifications in the circuit arrangements, as well as in the apparatus employed, may be made without departing from the scope of our invention as set forth in the appended claims.

What we claim is:

1. In combination in a multi-stage audio amplier including a pentode output tube having a rising frequency amplification characteristic, an inductive load in the anode circuit of said pentode tube having an impedance whose magnitude varies directly with frequency, and a resistance connected in parallel with said load, said resistance having a magnitude such as to limit the value of said load impedance.

2. In combination with means for amplifying electrical oscillations, which means includes a plurali-ty of electron discharge tubes, one of which tubes is a pentode output tube, said pentode having a tendency for strongly amplifying the high frequencies of the frequency band, an inductive load in the pentode anode circuit whose impedance varies directly with frequency, and means connected in parallel with the anode circuit load of said pentode tube whereby the high frequencies are amplified without any distortion.

3. An amplifier comprising a plurality of electron discharge tubes, one of said tubes being a pentode output tube, an inductive load in the anode circuit of the pentode tube adapted to produce distortion, and a resistance of a magnitude between 10,000 and 100,000 ohms connected in parallel with said load to reduce said distortion.

4. A multi-stage audio amplifier including a pentode output tube, means in the anode circuit of said output tube for negatively biasing the control electrode of said tube, an inductive load in the anode circuit of said pentode tube adapted to produce voltage variations which strongly react on the control electrode bias of the tube, and a resistance in parallel with said load to limit said variations.

5. In an audio frequency amplifier a power stage including an electron discharge tube provided with input and output circuits, an inductive load in the output circuit of said tube having an impedance whose magnitude varies directly with frequency, and means associated with said load and so proportioned with respect thereto as to substantially reduce the effect of impedance variation of said load on said output circuit, said tube comprising a cathode, anode, screen grid and auxiliary grid at cathode potential and having a rising frequency amplification characteristic.

6. In an audio frequency amplifier a power stage including a pentode output tube having a rising frequency amplification characteristic and provided with input and output circuits, an inductive load in the output circuit of said tube having an impedance whose magnitude varies directly with frequency, and means associated with said load and so proportioned with respect thereto as to substantially reduce the effect of impedance variation of said load on said output circuit, said inductive load comprising a transformer, and a reproducer coupled to said transformer.

'7. In an audio frequency amplifier a power stage including an electron discharge tube provided with input and output circuits, an inductive load in the output circuit of said tube having an impedance whose magnitude varies directly with frequency, and means associated with said load and so proportioned with respect thereto as to substantially reduce the effect of impedance variation of said load on said output circuit, said tube comprising a cathode, anode, screen grid and auxiliary grid at cathode potential and having a rising frequency amplification characteristic, said inductive load comprising a transformer, and a reproducer coupled to said transformer, and said means being connected across one of the transformer windings.

8. In an audio frequency amplifier a power stage including an electron discharge tube provided with input and output circuits, an inductive load in the output circuit of said tube having an impedance whose magnitude varies directly with frequency, and means associated with said load and so proportioned with respect theretoy as to substantially reduce the effect of impedance variation of said load on said output circuit, Said tube comprising a cathode, anode, screen grid and auxiliary grid at cathode potential and having a rising frequency amplification characteristic, said inductive load comprising a transformer, and a reproducer coupled to said transformer, and said means being connected across the transformer primary.

9. In an audio frequency amplifier a power stage including an electron discharge tube provided with input and output circuits, an inductive load in the output circuit of said tube having an impedance whose magnitude varies directly with frequency, and means associated with said load and so proportioned with respect thereto as to substantially reduce the effect of impedance variation of said load on said output circuit, said tube comprising a cathode, anode, screen grid and auxiliary grid at cathode potential and having a rising frequency amplification characteristic, said inductive load comprising a transformer, and a reproducer coupled to said transformer, and said means being connected across the transformer secondary.

10. In an audio frequency amplifier a power stage including an electron discharge tube provided with input and output circuits, an inductive load in the output circuit of said tube having an impedance whose magnitude varies directly with frequency, and means associated with said load and so proportioned with respect thereto as to substantially reduce the effect of impedance variation of said load on said output circuit, said tube comprising a cathode, anode, screen grid and auxiliary grid at cathode potential and having a rising frequency amplification characteristic, said load comprising a reproducer in the said output circuit, and said means comprising an impedance path across the reproducer.

ll. In an audio frequency amplifier a power stage including an electron discharge tube provided with at least a grid, cathode and anode, a source of anode current, means connected to said source, and common to the anode and grid circuits, for biasing said grid, an inductive load in said anode circuit adapted to produce voltage variations therein which react on the grid bias, and an impedance path in shunt with said load for substantially reducing said variations to minimize the reaction.

12. In an audio frequency amplifier a power stage including an electron discharge tube provided with at least a grid, cathode and anode, a source of anode current, means connected to said source, and common to the anode and grid circuits, for biasing said grid, an inductive load in said anode circuit adapted to produce voltage variations therein which react on the grid bias, and an impedance path in shunt with said load for substantially reducing said Variations, said path comprising a resistor of a magnitude between 10,000 and 100,000 ohms.

13. In an audio frequency amplifier a power stage including an electron discharge tube'provided with at least a grid, cathode and anode, a source ofanode current, means connected to said source, and common to the anode and grid circuits, for biasing said grid, an inductive load in said anode circuit adapted to produce voltage variations therein which strongly react on the grid bias, and an impedance path in shunt with said load for substantially reducing said variations,

said power tube additionally including a screen grid and auxiliary grid at cathode potential and having a rising frequency amplification characteristic.

14. In an audio frequency amplifier a power stage including an electron discharge tube provided with at least a grid, cathode and anode, a source of anode current, means connected to said source, and common to the anode and grid circuits, for biasing said grid, an inductive load in said anode circuit adapted to produce voltage Variations therein which react on the grid bias, and an impedance path in shunt with said load for substantially reducing said variations, said path including a resistor, and said load comprising a transformer.

15. In an audio power stage including a pentode output tube provided with a cathode, anode, screen grid, signal grid and auxiliary grid at cathode potential, said tube having a rising frequency amplification characteristic, a source of audio signal energy coupled to the signal grid, a loud speaker having its winding in the anode circuit of said tube and presenting a reactive load having an impedance whose magnitude Varies directly with frequency, a source of energizing potential for the tube electrodes, and a resistor connected between the positive terminal of said potential source and the anode to reduce the effect of impedance variation of said reactive load on the anode circuit, said resistor being in shunt with said speaker.

16. In an audio power stage including a pentcde output tube provided with a cathode, anode, screen grid, signal grid and auxiliary grid at cathode potential, said tube having a rising frequency amplification characteristic, a source of audio signal energy coupled to the signal grid, a loud speaker having its winding in the anode circuit of said tube and presenting a reactive load having an impedance whose magnitude varies directly with frequency, a source of energizing potential for the tube electrodes, a resistor connected between the positive terminal of said potential source and the anode to reduce the effect of impedance variation of said reactive load on the anode circuit, said resistor being in shunt with said speaker, and a connection between the screen grid and the positive side of said resistor.

EKKO OOSTERHUIS. CORNELIS HENDRIK MOREL. 

